The present invention relates to a closure for sample vials or vessels for introducing samples into a gas chromatograph utilizing the vapour space (head space) method and particularly relates to a closure for sample vials of the type having a rubber disc retained within a metal cap having a central aperture, the metal cap including a rim deformed about a circumferential bead at the neck of the sample vial.
In the vapour space method, hereafter called head space method, of introducing a sample into a gas chromatograph, a sample vial is filled with a sample liquid and is closed at the top by a self-sealing rubber disc or septum. The sample vial is then heated to a predetermined temperature. The injection section of the gas chromatograph includes a needle which, by means of a suitable mechanism, pierces the rubber disc and extends into the head space above the level of the liquid in the sample vial. The needle lies in communication with the separating column inlet of the gas chromatograph as well as with a source of carrier gas. A shut-off valve is provided intermediate the separating column inlet and the needle and carrier gas source. Initially, the shut-off valve is opened and carrier gas passes through the needle into the head space above the liquid level of the sample liquid until the pressure of the carrier gas source is obtained within the head space. Under pressure equalization, carrier gas flow is stopped for a predetermined period of time by closing shut-off valve. As a result, the pressure at the separating column inlet decreases and the excess pressure prevailing in the head space causes sample vapour and carrier gas to flow from the head space in the sample vial through the needle toward the separating column inlet. The partial pressures within the head space of the respective components in the sample are proportional to the concentration of the components in the liquid sample. Consequently the chromatogram of the head space enables identification of the concentrations of the respective components in the sample.
Conventional sample vials for head space analysis have closures including rubber discs formed of butyl rubber. Butyl rubber discs, however, are neither sufficiently temperature stable nor satisfactorily chemically inert. For example, at temperatures above 80 degrees centigrade, butyl rubber discs give off volatile components which generate a background in the chromatogram and render any high sensitivity measurements impossible. Furthermore, butyl rubber discs have a high permeability for non-polar organic compounds, e.g. hydrocarbons, and consequently the composition of the sample enclosed in the sample vial may change.
To avoid these difficulties, rubber discs made from temperature stable silicon rubber have been utilized for the septum. These discs are laminated with a thin polytetrafluoroethylene layer on the side facing the interior of the sample vial. The silicon rubber seals about the needle upon penetration and during the metering operation and is self-sealing after removal of the needle from the sample vial. The silicon rubber also provides the required mechanical strength. The thin polytetrafluoroethylene layer, e.g. having a thickness of 0.05 to 0.1 mm, forms a chemically inert barrier layer for preventing entry of volatile silicon rubber components into the sample as well as outward diffusion of volatile sample components through the silicon rubber. Usually, these rubber discs are retained on the sample vial by a metal cap having a central aperture, the metal cap having a rim deformed to grip about a circumferential bead at the neck of the sample vial.
The foregoing described closures, however, are not sealed against high pressures since the rubber discs, while retained by the metal caps, are not pressed against the necks of the sample vials. The usually flat rim of the vial is not sufficiently planar to form a high pressure seal. Rather, it has grooves or flutes due to its manufacturing process which preclude sealing at high pressures.
It is desirable however, to utilize the head space method within a fairly wide range of temperatures, for example up to 190 degrees centigrade. Increased pressures will thus be obtained in the sample vial. Care must be taken to ensure that the closure of the sample vial is tightly sealed to the vial at higher temperatures and pressures. Also, it is necessary to ensure that the limiting strength of the vial is not exceeded so that the vial will not burst or explode because of faulty operation. This may occur, for instance, by placing a sample vial enclosing an aqueous sample into the heating apparatus and heating the sample within the sample vial to temperatures above 190 Centigrade. At these temperatures, the pressures within the sample vial will be about 12 bar and higher. It will readily be appreciated that bursting a sample vial may cause damage to the apparatus as well as expose the operator to danger.